Sizes returned by filesizeformat are incorrect.

[Nathan Osman]

The template filter 'filesizeformat' appears to be returning incorrect results:

>>> from django.template.defaultfilters import filesizeformat
>>> filesizeformat(123456789)
u'117.7 MB'

According to ​the International System of Units (SI), the prefix "mega-" indicates a quantity of exactly 1000000. Django appears to be using 1024*1024 (1048576) to represent a megabyte which corresponds with the SI prefix "mebi-".

This error can be corrected in one of two ways:

• Replace the units in the strings returned with their proper equivalents:

  KB => KiB
  MB => MiB
  etc.
  

• Use the correct values for KB, MB, etc.:

  1024    => 1000
  1048576 => 1000000
  etc.
  

[Russell Keith-Magee]

Duplicate of #8733, and I haven't seen any significant change in the way the world views the SI prefixes since that ticket was closed wontfix.

[Claude Paroz]

I can buy the rationale of #8733, but this should be documented.

[Claude Paroz]

Here's the text I suggest to add to the filesizeformat filter documentation. This should be revised/adapted by a native English speaker.

.. admonition:: File sizes and SI units

    Strictly speaking, ``filesizeformat`` does not conform to the International
    System of Units, which recommends to use KiB, MiB, GiB,... when byte sizes
    are calculated in powers of 1024 (which is the case here). Instead, Django
    has chosen to keep using traditional unit names (KB, MB, GB,...)
    corresponding to the more widespread usage.

[Tim Graham <timograham@…>]

In 63e6cd1bb3edccb4db1eb6a79153fa31d2114cd4:

Fixed #19348 - Clarified the units of filesizeformat.

Thanks george_edison for the report and Claude Paroz for the patch.

[Tim Graham <timograham@…>]

In c90156fc188200795a24c24b9ea97eef32356e55:

[1.5.X] Fixed #19348 - Clarified the units of filesizeformat.

Thanks george_edison for the report and Claude Paroz for the patch.

Backport of 63e6cd1bb3 from master